Various type ceramic closures are already known to close each end of the tubular light-transmitting ceramic envelope used for sodium vapor lamps such as described in U.S. Pat. Nos. 4,034,252 and 4,065,691, both assigned to the assignee of the present invention. In said earlier issued patent, the end of said arc tube is closed with a ceramic plug that is inserted into the arc tube end and which has a central aperture to permit passage of an electrode inlead therethrough, with all of said members being hermetically sealed together by sealing frit. A metal reflector band is also mentioned in said patent for placement at the upper end of said arc tube in order to help maintain the desired temperature of lamp operation. The sealing frit is said to comprise primarily aluminum oxide and calcium oxide and which is heated to provide a molten glass during the sealing operation. In the subsequently issued aforementioned patent, the end closure is provided by a ceramic plug inserted into the end of the arc tube and which again permits passage of an electrode inlead wire therethrough, and with a hermetic seal being affected between all of said joined closure members. A metal reflector band is also mentioned in said patent for the same utilization above described.
Difficulties are still being experienced with the known end closures in providing reliable hermetic seals between the metal and ceramic lamp components being joined. For example, sealing the ceramic plug to the ceramic arc tube has proven difficult with success generally requiring careful control of both disc and tube dimension along with precise attention being given to the amount of sealing frit used. Too little sealing frit produces incomplete filling of the surfaces being joined and with leakage occurring at such locations. Too much frit produces large sealing fillets that can cause cracking of the ceramic tubing material due to any slight mismatch in thermal expansion between the sealing frit and the ceramic tubing. Additionally, the insertion of a ceramic plug into the end of the arc tube requires an increase in tubing length to maintain the same inner volume of the arc tube. Such construction not only increases cost of the lamp being manufactured but has further required the aforementioned head reflector means in order to preserve the same lamp operating conditions. It may be desirable, therefore, to improve the end closure for this type lamp without undue modification or additional cost in either the lamp manufacture or subsequent lamp operation.